World No 1 and defending champion Novak Djokovic marched imperiously into the second round with a dominant 6-3, 6-2, 6-4 victory over Chung Hyeon.

The 28-year-old Serb looked as untroubled by the Melbourne heat as he was by the South Korean teenager in the 115-minute contest, charging around Rod Laver Arena and rattling off 40 winners.

World number 51 Chung showed he had some impressive weapons at his disposal, particularly in the opening set, but was not always able to convert his best shots into points.

The 19-year-old stalled Djokovic's progress in an absorbing fifth match of the second set, holding serve after eight deuces, but the five-times champion soon advanced to a second round date with French teenager Quentin Halys, who beat Croatia's Ivan Dodig 6-4, 6-7(4), 6-4, 7-5.

Kazakhstan's Yulia Putintseva pulled off the first big upset at the Open, defeating former world No 1 Caroline Wozniacki.

Putintseva won a marathon second set to beat the Dane 1-6, 7-6, 6-4 at Hisense Arena, and advance to the second round.

This is the first time the 16th seed was ousted in the first round of the Australian Open.

Meanwhile, seventh-seed Kei Nishikori also advanced to the second round after beating Philipp Kohlschreiber of Germany 6-4, 6-3, 6-3.

IMAGE: The United States' Serena Williams during her first round match against Italy's Camila Giorgi. Photograph: Quinn Rooney/Getty Images

Defending champion Serena Williams reached the second round after a tense 6-4, 7-5 win over feisty Italian Camila Giorgi in her first tour match in four months.

The world number one had barely played any competitive tennis since her US Open exit last September and entered a sweltering Rod Laver Arena with doubts over her fitness to face the highest-ranked unseeded player in the draw.

The slight but hard-hitting Giorgi conceded a 4-1 lead early but steadied to give the American a genuine test and Williams's roars of frustration underlined the growing tension as the match progressed.

However, it was Giorgi who wilted at 5-5 in the second set, a double-fault handing Williams the decisive break and allowing the American to close out the match with a barrage of booming serves.